As for personal experience, well over two decades in the business, if we're keeping score.
So, you would have readers believe that developers are egoless automatons who do everything by the book? If true, they'd be the only faultless profession on the planet.
15 years with software dev experience and every single dev team at midsize and larger companies has always asked for far more time than necessary. Same with almost all enterprise consultants.
Or did you not give them the time they asked for and declare yourself correct after squeezing in a deadline? And if so how do you know what corners were cut or what technical debt was accrued to make a deadline the team did not sign off on?
Whether every single person is noble is irrelevant. People are making broad generalizations here against the reputation of developers with what I see to be weak and anecdotal evidence.
If someone has made serious inquiry into the subject and controlled for complexities like I suggested above then by all means let the chips where they may.
This is human nature, people will look out for themselves. Many, if not most, are looking to do the least amount of work in return for the largest reward, and it's very easy at bigger companies to pad hours, compounded by bureaucracy and poor management. This is seen in many other professions amongst all types of people, nothing specific to software.
Could I do the work myself in a week? Yes. Can I assign it to a team of diverse junior to mid-level developers and expect them to do it in a week as well? Realistically, no. They have to investigate, acquire knowledge, work together, be guided on the correct design (or get to it themselves), and then finally do the work and test it to sufficient quality.
Even more so the case if it's tech we have not worked with before.
This is my concern. It absolutely doesn't give you that knowledge. Accurately estimating development of non-trivial software is still an open problem. The best solution we have to date is to accept this fact and use it in our process and planning to our advantage when possible.
If the work you're talking about is rote and mostly a matter of repeating a pattern rather than innovating and engineering, sure I can see that being more predictable. Unfortunately quite a bit of software development does not fall into that bucket.